Embodiments in accordance with the present invention relate to a technique of improving recognition capability of servo timing mark (STM) in a magnetic disk system, and furthermore relates to a technique of improving recognition capability without increasing bit length of the STM.
In a magnetic disk system employing a data surface servo system (sometime called sector servo system or embedded servo system), servo sectors and data sectors are alternately arranged in a circumferential direction. When a data sector is accessed, a position of a magnetic head is controlled according to a signal reproduced from a servo sector. While the servo sector is indispensable for position control of the magnetic head in the magnetic disk system in the data surface servo system, since it is an usable region for recording user data, it desirably has a small area to the utmost.
In a servo sector, a pattern for recognizing a position of the servo sector is written, which is known as servo timing mark (STM) or servo address mark (SAM). When a system can not recognize the STM, the system is into a servo unlock state in which it can not perform servo control. Previously, when a system was into the servo unlock state, the system continuously opened a search window to operate a servo control circuit until it was able to detect the STM. Even if the search window was continuously opened, a unique pattern was selected for the STM so that the STM was able to be distinguished from all data patterns recorded in a magnetic disk, therefore the system was able to recognize the STM distinguishingly from reproduced servo data other than the STM and user data, and return into a servo lock state.
[Patent literature 1] JP-A-2000-311447.
[Patent literature 2] JP-A-8-31123.
[Patent literature 3] JP-A-2002-150728.
Regarding such a method, patent literature I discloses a technique that a detection circuit of an address mark (AM) continuously detects an AM pattern at start, and once the circuit detects the AM, it determines a detection range of the AM and detects the AM only within the range, so that false detection is prevented. The literature further describes that when the AM pattern is not detected in the detection range of the AM, the circuit is into a mode of continuously detecting the AM. Patent literature 2 discloses a technique that even if detection error of the address data of the servo occurs, AGC operation is performed from leading of servo information in a subsequent sector while counting timing by a counter. Patent literature 3 discloses a technique that the search window is temporally expanded to prevent occurrence of detection error of servo information.
In a technical field of servo control, in order to reduce areas occupied by the servo sectors in a recording surface of the magnetic disk, efforts of increasing frequency of servo data or decreasing a bit number of the servo data have been previously made. As sampling frequency of the servo data approaches frequency of user data by increasing the frequency of the servo data, when the system is into the mode of continuously detecting the STM in the servo unlock state, a situation that user data or servo data other than STM are falsely recognized as STM tends to occur. The same problem may occur when discriminating power of STM as a pattern is reduced as a result of decreasing the bit number of the servo data.
The detected STM is used as information indicating a reference position of each servo sector. Since the system performs servo control while recognizing a bit string detected following the STM as address information or positional information of the servo data, unless the system detects the STM at an accurate position, it can not perform the servo control. Thus, an object of embodiments in accordance with the present invention is to provide a magnetic disk system that is reduced in possibility of false detection of falsely recognizing other data as STM. Another object of embodiments in accordance with the present invention is to provide a magnetic disk system in which possibility of false detection of STM can be decreased without enlarging the area of the servo sector even if frequency of the servo sector is increased. Still another object of embodiments in accordance with the present invention is to provide a magnetic disk system in which possibility of false detection of STM can be decreased even if the bit number of the servo sector is decreased. Still another object of embodiments in accordance with the present invention is to provide a reproduction method of the servo data in such a magnetic disk system.